Diluted bull semen samples were irradiated with 180 kv X-rays. Dose response curves were measured for the survival fraction of the spermatozoa, and for the average velocity of the surviving cells. The dose response curves did not show a sensitivity threshold. The half-value dose was determined as 11 kr for the survival fraction and 10 kr for the average velocity. Target theory was adapted specially to explain the form of the measured dose response curves. From this target theory it was found that a small sensitive element is present in the sperm cell with a volume of approximately 0.7(5) x 10(-15) cm(3).